Two men were stabbed during the fight and bystanders were knocked to the ground as the brawl raged the length of the 60-metre pool on a sweltering summer’s day last year, a court heard.

Parents among the 1,000 bathers had to pluck their children from the Parliament Hill Lido as they tried to flee what one witness described as a “riot” involving up to 20 young black men.

Lifeguard Greg Stechman, who tried to break up the fracas, said: “There was panic with people running backwards and forwards to get their children out of the pool. People were getting knocked out of the way.”

Last Thursday, police were forced to admit in court that they had lost important CCTV footage of the incident.

Det Con Roger Powell, the officer in the case, told the court: “I went to the scene on the day of the fight, viewed the CCTV and asked for it to be downloaded.

“It was downloaded, but the bit involving the fight was not downloaded.”

Defence counsel Patrick Harte said: “This crucial bit of CCTV never made its way into this case.”

Police said the CCTV footage was viewed by officers at the scene and claim it was “unlikely to have altered the outcome of the case”.

A spokeswoman said: “Officers who had previously viewed the footage in full confirmed that although the missing section showed a disturbance it did not clarify what happened or who was involved.

“Therefore, it is unlikely that it would have altered the outcome of the case.”

Suma Brown, who had gone to the outdoor pool with friends on July 24 last year, was accused of one count of affray.

Appearing at Blackfriars Crown Court, he admitted throwing up to three punches in self-defence. The 20-year-old said he had become embroiled in the fight to help his friend, who had been set upon by a gang of up to six men at around 5pm.

Mr Brown, who is studying business management at the University of East London, said: “I punched him because he was about to hit my friend, so I was defending my friend.”

In a police interview last year Mr Brown, from Bourne Terrace in Little Venice, told detectives he had not known his assailants and the attack appeared to be random.

One of his friends had been stabbed in the abdomen and another man was stabbed in the buttocks during the attack.